The effect of low T-g polybutadiene (PBD) rubbery polymer as a part of polysiloxane/PBD E-glass fibers sizing on mechanical properties of a corresponding epoxy composite material has been evaluated by the punch shear test technique. The results show that the use of hydroxyl terminated PBD led to significant increase in interface shear strength, energy absorption, as well as dynamic modulus and T-g in a corresponding composite material. The sizing composition and fiber morphology were characterized by FTIR spectroscopy and AFM microscopy, respectively. Possible rational for such sizing composition! material property relationship is discussed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 841-848, 2010